1. Field of the Invention
The present invention relates to a control device for a servomotor used as a driving source of a machine tool.
2. Description of the Related Art
It is often the case that a control device for controlling and driving a servomotor used as a driving source of a machine tool is generally constituted of three control loops, namely, a position control loop, a speed control loop, and a current control loop.
FIG. 7 is a block diagram schematically illustrating a configuration of a general control device for a servomotor. In the following, constituent elements indicated with the same reference numerals throughout the drawings are constituent elements having substantially the same functions.
In a position control loop, a speed command generation unit 131 generates a speed command value, based on position information of a servomotor 2 detected by a position detection unit 142 mounted on the servomotor 2 and based on a position command value generated by a position command generation unit (not illustrated). In a speed control loop, a current command generation unit 121 generates a current command value, based on speed information of the servomotor 2 detected by a speed detection unit 141 mounted on the servomotor 2 and based on a speed command value generated by the speed command generation unit 131. In a current control loop, a driving command for driving a power converter 3 using a compensator 111 is generated, based on a detection value of a driving current of the servomotor 2 and based on the current command value generated by the current command generation unit 121. The power converter 3 is an inverter configured to convert direct current power to alternate current power by a switching operation of a switching element provided in the power converter 3, for instance. The power converter 3 is configured to control a conversion operation of converting direct current power to alternate current power for driving the servomotor 2 by controlling a switching operation of the switching element in the power converter 3 in response to receiving a driving command. The servomotor 2 is operated by using alternate current power output from the power converter 3, as a driving power. Therefore, controlling the alternate current power output from the power converter 3 makes it possible to control and drive the speed of the servomotor 2, the torque of the servomotor 2, or the position of a rotor. A movable unit of a machine tool is driven by driving the servomotor 2.
As a compensator used in a current control loop, a PI compensator, and an I-P compensator are known.
A PI compensator has an advantage in shortening a rise time compared to an I-P compensator. Generally, a shortened rise time of a current control loop makes it possible to set a high gain of a speed control loop formed on the outside of the current control loop. Further, setting a high gain of a speed control loop makes it possible to reduce an error (position error) with respect to a position command value in a machine tool. PI control and I-P control have the same oscillation limit. Therefore, when a current control loop gain is determined with a margin substantially the same as each other from the oscillation limit with respect to a PI compensator and with respect to an I-P compensator, PI control can be operated at a higher speed, and a position error of PI control can be reduced, as compared with I-P control. Accordingly, using a PI compensator in a current control loop contributes to enhancement of cutting precision of a machine tool.
On the other hand, an I-P compensator has an advantage in reducing an amount overshooting of a machine tool compared to a PI compensator. Therefore, using an I-P compensator in a current control loop makes it possible to stably position with a reduced amount of overshooting in a machine tool.
For instance, as disclosed in U.S. Pat. No. 3,628,199, there is a technique that enhances both the response and stability in servomotor control by appropriately using a PI compensator and an I-P compensator, as a compensator in a current control loop, taking into account of the aforementioned characteristics of the PI compensator and the I-P compensator. According to the invention disclosed in U.S. Pat. No. 3,628,199, gain values of a current control loop and a speed control loop are adjusted, as necessary, by respectively using an I-P compensator in positioning mode (non-locus control mode) such as a fast forward operation, and a PI compensator in locus control mode such as cutting and feeding operation mode, as a compensator in a current control loop. Specifically, securing a high gain by using a PI compensator in locus control mode makes it possible to enhance response of a current control loop, and also makes it possible to enhance response of speed control and position control to thereby enhance cutting processing precision of a machine tool while enhancing control performance. Further, securing a low gain by using an I-P compensator in non-locus control mode makes it possible to stably secure control.
In the invention disclosed in U.S. Pat. No. 3,628,199, only the compensators are switched depending on whether the servomotor is in locus control mode or in non-locus control mode. For instance, when the driving mode of the servomotor is switched from non-locus control mode to locus control mode, the compensator to be used in a current control loop is switched from an I-P compensator to a PI compensator. In this case, although response of the current control loop enhances, a gain at a mechanical resonance point increases in a high frequency range. Specifically, when a PI compensator is used in a current control loop, a gain in a high frequency range increases compared to a configuration of using an I-P compensator. As a result, vibrations that are not generated when using an I-P compensator may be generated when using a PI compensator.
The invention disclosed in U.S. Pat. No. 3,628,199 has an object of enhancing control performance by using a PI compensator in locus control mode to thereby enhance processing precision of a machine tool, but does not consider the response characteristics in a high frequency range, and does not take any measures against avoiding resonance when using a PI compensator in a current control loop in locus control mode. In other words, the invention disclosed in U.S. Pat. No. 3,628,199 appears to be made based on the premise that a machine has no mechanical resonance point. Actually, however, an elastic body with a mass has a natural frequency. Therefore, a machine without a mechanical resonance point does not exist.
As has been described above, according to the invention disclosed in U.S. Pat. No. 3,628,199, although use of a PI compensator in a current control loop remarkably enhances response to thereby enhance processing precision of a machine tool, stability of control performance of the machine tool may be deteriorated.